Urban Wind-heat Environment Driven By Heat Island Circulation Under Calm Wind Condition

The rapid urbanization makes the urban population increase and the building density increase.The high-density buildings worsen the Urban Ventilation and strengthen the heat island effect.This leads to the spread of pollutants and epidemics in cities.Nowadays,air pollution and extreme heat wave seriously threaten the safety and comfort of life,so it is urgent to improve the urban ventilation.Accurate assessment of urban ventilation capacity and thermal environment is very important for making reasonable urban environmental planning and improving urban air environment.In the past,more attention has been paid to wind pressure driven ventilation,while less attention has been paid to heat pressure driven ventilation in cities.Heat pressure driven ventilation is also an important ventilation mechanism under weak wind conditions.In the third chapter,the atmospheric junction is introduced through the improved multiscale computational fluid dynamics(CFD)algorithm,and the flow mechanism of urban heat island circulation is studied through numerical simulation aiming at the divergence of the structure of urban heat island circulation in gas chamber and water tank experiments.By changing the intensity of atmospheric stratification and heat source in the model,the representative plume model observed in the gas chamber experiment is successfully simulated in the numerical calculation.The occurrence of this phenomenon is quantitatively studied,and the effects of atmospheric stratification and heat flow intensity on the transient plume model are discussed in detail.The simulation results show that the higher the heat flow intensity is,the more obvious the unstable state of the thermal plume is;and the enhancement of the atmospheric stability will inhibit the unstable state and make it transform into a stable state.Through numerical simulation,the influence of the unsteady state of thermal plume on the diffusion of gaseous pollutants in the city is also discussed.The numerical results show that when pollutants are released from one side of the city,the periodic oscillation of the heat plume increases the probability of pollutants from one side of the city entering the other side of the city.In the stable plume stage,the pollutant crossing rate is about 0.3.If the thermal plume changes to the unstable state,the pollutant crossing rate will continue to increase and approach to 0.5.In Chapter 4 and Chapter 5,for the problem of Urban Ventilation driven by thermal pressure,this paper proposes a model which combines the idealized high Reynolds number porous media urban model and multi-scale CFD method to quantitatively study the Urban Ventilation(breathing)capacity under the condition of calm wind,and studies the influence of urban heat island intensity,urban ventilation(breathing)capacity and urban air quality Factors and improvement measures.Based on the air exchange rate(ACH),age of air and PFR,the contribution of five common factors to urban heat island and air quality was analyzed qualitatively and quantitatively.The numerical results further show that: 1)for the intensity of heat island,the main influencing factors are: urban heat flux > urban scale > temperature stratification > urban density > building height.2)In terms of air quality,the importance of influencing factors is as follows: building height > heat flux > urban scale > temperature stratification > urban density.Urban heat flux has great influence on urban heat island intensity and air quality.The increase of urban heat flux will increase the ventilation performance of the city,but also will increase the intensity of heat island.The increase of atmospheric stability limits the environmental capacity,which is not conducive to Urban Ventilation and thermal evacuation.The increase of urban scale will increase the intensity of heat island and worsen the air quality.The age of air is greatly deteriorated with the increase of urban building height,which has little effect on the intensity of heat island.The increase of urban density(excluding the change of heat emission)has the weakest effect on buoyancy driven flow,but it will slightly worsen air quality and heat island intensity.The model and research results of this paper have certain theoretical significance for municipal departments and urban planners to alleviate urban heat island and urban air pollution through reasonable planning of urban building layout.